Transistor servo amplifier with dither



March 15, 1966 H. B. WATTSON 3,241,016

TRANSISTOR SERVO AMPLIFIER WITH DITHER Filed Nov. 9, 1962 PRE AMPLIFIERIG DISCRIMINATOR l2 DITHER GENERATOR \F ,sa '1 Il /ii I l a! 54 a I 5 I L I l HYDRAULIC 3X SERVO MECH.

/ sow OUTER TRANSFORMER LOOP S ON AIRCRAFT PISTON 5 CONTROL A SURFACE 1N VEN TOR HARRY B. WATTSON 977' OQA/E Y United States Patent Ofifice 3,241,015 Patented Mar. 15, 1966 3,241,016 TRANSISTOR SERVO AMPLIFIER WITH DITHER Harry B. Wattson, Rutherford, NJ., assignor to The Bendix Corporation, Teterboro, N.J., a corporation of Delaware Filed Nov. 3, 1962, Ser. No. 236,619 13 Claims. (Cl. 318-48) This invention relates to amplifiers and more particularly to a servo amplifier having a dither generator for providing accurate setting of servo nulls.

Where high accuracy of servo nulls is required, particularly as applied to torque motors positioning valves on hydraulic servos of a type shown in US. Patent No. 2,938,503, granted May 31, 1960, to Howard H. Laucks, and assigned to The Bendix Corporation, present day servo amplifiers are generally inadequate in that they suffer from inaccurate settings of the nulls for the hydraulic servo. This is due to static friction inherent in the hydraulic valves positioned by torque motors driven by the servo amplifiers. One method for increasing accuracy of such servo nulls is by applying a dither signal to the torque motor, enabling the motor to oscillate with minute amplitudes of oscillation about the null of the hydraulic servo. However, circuits for producing dither in the past have been unable to withstand shock and vibration, and have required large amounts of power for operation. These are obvious drawbacks of such circuits which otherwise can be especially useful in missile applications because of the accurate null settings.

An amplifier constructed according to the present invention provides an output signal of correct amplitude and direction for operating a torque motor which may operate port valves of a hydraulic servo. Although designed primarily for such use the circuit has other applications such as driving a magnetic amplifier in order to operate a large motor, operating a relay where a reversing DC. signal is required, or even operating a small motor directly.

Therefore, one object of this invention is to provide a servo amplifier with dither having low power consumption and high ability to withstand large amplitudes of vibration or shock.

Another object of this invention is to provide a servo amplifier with dither having low heat dissipation, long life, small size, and light weight.

Another object of this invention is to provide an amplifier producing a clean, well-phased output signal wherein a signal from an input chain is amplified by a preamplifier.

The invention contemplates a discriminator, means applying a control signal to the discriminator, and a dither generator applying an oscillating signal to the discriminator whereby the discriminator applies an output signal of varying amplitude and polarity combined with a dither signal to the windings of a torque motor for control of the port valves of a hydraulic servo.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

In the drawing, the single figure is a circuit diagram showing the servo amplifier with dither embodying the present invention.

Referring to the figure, numerals and 10A represent windings for controlling the direction of deflection of the shaft of a torque motor 11 of conventional type producing a mechanical torque in one sense or in an opposite sense dependent upon the selective energization thereof to control hydraulic port valves of a hydraulic servo mechanism 11A of a type shown in the US. Patent No. 2,938,503. The windings 10 and 10A are so connected as to selectively receive an output signal from a discriminator 12 through terminals FF and GG of a terminal strip 13. A milliammeter 14 is connected in series with motor winding 10, and a milliammeter 15 is connected in series with motor winding 10A. These milliammeters for example have scale readings of 0-25 ma. Discriminator 12 receives a control signal from a preamplifier 16 through a coupling transformer 17. A dither generator 18 applies a dither signal, which consists of small amplitude oscillations, to the discriminator 12 through a coupling impedance consisting of a resistor 19 and a capacitor 20. A variable resistor 21 is used for adjusting dither amplitude to a value which provides accurate setting of the servo nulls of the hydraulic servo 11A without excessive rotational overshoot of the torque motor 11.

Preamplifier 16 receives two signals at a signal input 22, which comprises the emitter of a transistor 23. Each signal is received through a different one of a pair of isolation resistors 24 and 25. Excitation voltages to the command synchro 31, follow up synchro 34 and feedback Schaevitz unit or condition responsive variable transformer 37 are applied to three input terminals 26, 27, and 28 and suitable ground connections by excitation transformer 9 connected to A.C. power source 8. Terminal 26 is connected through a phasing resistor 29 to an adjustably positioned rotor winding 30 of a synchro 31. Angular position of rotor 30 may be controlled by an operatoroperative knob 7 or other suitable control means such as a directional or vertical gyroscope or controlling instrument. The angular position of the rotor 30 corresponds to the desired input signal for the command synchro 31. A second voltage is coupled from terminal 27 through a phasing resistor 32 to an adjustably positioned rotor winding 33 of synchro 34. This second voltage may provide through the synchro 34 a follow up signal from an outer follow up loop of a servo system, including an outer loop hydraulic piston controlled by a valve in turn positioned by an inner loop piston 112 controlled by servo mechanism 11A, and which follow up signal depends upon the angular position of rotor 33 effected by outer loop piston 100. The voltages applied to terminals 26 and 27 may, for example, be of 12 volts amplitude single phase A.C. The piston 100 may be operatively connected so as to position an aerofoil surface such as a rudder, elevator or aileron 101.

A third input voltage is applied through a phasing resistor 35 to the primary 36 of a condition responsive transformer 37. Therefore, the third input voltage provides through the condition responsive variable transformer 37 a feedback signal depending upon the sensed condition, from a second or inner loop of the servo system in which an adjustable magnetic core 114 is positioned by the inner loop hydraulic piston 112 to provide a feedback signal tending to retard oscillation of the controlling system. The voltage applied to terminal 28 may, for example, be of 26 volts amplitude single phase A.C.

Output of transformer 37 is taken from its secondary winding 38 and applied through terminals BB of terminal strip 13 to the isolation resistor 25. This provides one input signal to the preamplifier 16. A potentiometer 39 having a variable tap 40 is connected across two stator windings 41 and 42 of follow-up synchro 34. The tap 40 is connected to one side of a stator winding 43 of synchro 31. A stator winding 44 of synchro 31 is connected through terminals AA of terminal strip 13 to isolation resistor 24, providing the second input signal to preamplifier 16. Potentiometer 39 provides a convenient means for control of output amplitude of synchro 34.

By interconnecting the stator windings of synchros 31 and 34-, the amplitude of signal applied to resistor 24 is the algebraic sum of the voltage cross windings 43 and 44, plus the voltage across windings 41 and 42 reduced by a factor determined by the setting of potentiometer 399. The stator voltages of synchros 31 and 34 are dependent upon the angular settings of rotors and 33, respectively.

A power supply 45 is connected to a convenient source of A.C. power 8, of, for example, 115 volts 400 cycles, through terminals 46 and 47. With such input, the power supply may, for example, provide outputs of 12 volts single phase A0. at a terminal 49, and 115 volts single phase A.C. at a terminal 50. A third terminal, 48, is grounded. The 115 volt output is coupled through a step-down transformer 51, across a common point 52 between windings 10 and 10A of the torque motor 11 and a common point 53 connecting bases 54 and 55 of a pair of transistors 56 and 57, respectively, utilized in discriminator 12. Common point 53 is connected to transformer 51 through terminals ll of terminal strip 13. The 12 volt signal from terminal 49 of power supply 45 is coupled to the emitter 59 of a transistor 66 in dither generator 18 through terminals HH of terminal strip 13 and an isolation capacitor 58 connected in series. Capacitor 58 prevents any DC. voltage which may appear on emitter 59 of transistor 66 from being shorted out through terminal 49 of the power supply 45.

Output of discriminator 12 is provided by a pair of collectors 61 and 62 of transistors 56 and 57, respectively. Input to the discriminator 12 is coupled from the secondary winding 77 of transformer 17 to emitters 63 and 64 of transistors 56 and 57, respectively, through a pair of coupling resistors 65 and 66, respectively. The secondary winding of transformer 17 is centertapped to ground. Output of the dither generator 18 is applied across resistor 66 from the potentiometer 21. This has the efiect of putting the dither generator 18 output on the emitter of a single discriminator transistor, even though output of the preamplifier is applied to both emitters of the discriminator transistors Working in a push-pull arrangement. A DC. bias, of for example 22.5 volts positive, is applied from an emitter bias supply 67 through a milliammeter 68 and terminals EE of terminal strip 13 to a common point 69 between a pair of emitter bias resistors 70 and 71. The other sides of resistors 76 and '71 are connected to emitters 63 and 64 of transistors 56 and 57, respectively. Milliammeter 68 for example has a range of 0-10 MA. A collector bias supply 72, providing for example an output of 45 volts negative, is connected through a milliammeter '73, terminal DD of terminal strip 13, and a dropping resistor 75, to the collector 74 of transistor 60. Bias supply 72 is also connected to the collector 76 of transistor 23 in preamplifier 16 through the primary winding 78 of transformer 17. Milliammeter 73, for example, has a range of 0-40 MA. A base resistor 80 is connected between the base 79 of transistor 23 and ground.

Dither generator 18 includes a loading resistor 31 connected in series with a parallel-connected combination of a resistor 82 and capacitor 83 between emitter 59 of transistor 60 and ground. Variation of size of resistor 82 and capacitor 83 changes the frequency of oscillation of the dither generator. Loading resistor 81 controls the bias amplitude applied to emiter 59 of transistor 60. A resistor 84 is comiected between the base 85 of transistor 66 and ground. This resistor promotes negative resistance in the dither generator circuit, which is necessary for oscillation of the dither generator 16 as is well known in the art. A capacitor 86 is connected between collector 74 of transistor 60 and ground, and has the function of determining oscillation frequency of the dither generator 18 in a similar manner as capacitor 83.

In operaton, assume the windings of rotors 36 and 33 are properly excited and transformer 37 is also properly energized. A signal of magnitude dependent upon the angular positions of rotors 30 and 33 is then applied from the synchro stators to resistor 24, and a signal dependent upon the condition sensed by transformer 37 is applied from the secondary of the transformer to resistor 25. The amplitude of voltage applied to emitter Z2 is then substantially equal to the algebraic sum of voltage appearing across the combination of resistors 24 and 25 multiplied by the ohmic value ratio of resistor 25 to resistors 24 plus 25. The voltage is amplified by preamplifier 16 and is applied across the primary 78 of transformer 17. The transformed voltage is then taken from the secondary winding 77 and applied to emitters 63 and 64 of transistors 56 and 57, respectively, through resistors 65 and 66, respectively. Because the secondary winding 77 is centertapped to ground, the signal appearing at one side of the secondary winding with respect to ground will be of opposite polarity and out of phase with the signal on the other side of the secondary winding with respect to ground. Thus the signals applied to emitters 63 and 64 will be 180 out of phase with each other, and of opposite polarities.

The signal applied to emitter 64 of transistor 57 is modulated by the oscillation of dither generator 18 which are applied across resistor 66 with amplitude controllable by setting of potentiometer 21. Modulation takes place in resistors 21 and 66. Transistors 56 and 57, functioning as a pair of amplifiers connected in push-pull configuration, amplify the signal appearing on their emitters 63 and 64, respectively, with respect to common point 53. Thus the signal appearing at collector 61 of transistor 56 is applied to winding 16 of the torque motor 11 and the signal appearing on collector 62 of transistor 57 is applied to winding 10A of the torque motor 11. Thus, it is obvious that winding 16A receives the dither generator 18 signal in addition to the preamplifier 16 signal. This causes minute oscillation of the torque motor' 11 which enables the motor 11 to overcome static friction in port valves of the hydraulic servo 11A and provides a more accurate null for the output shaft of the hydraulic servo 11A. It is also obvious that a change in the position of command rotor 36 will produce a new signal at emitter 22 of transistor 23 in the preamplifier 16, thereby changing the signal applied to discriminator 12 and in turn changing the signal applied to windings 10 and 10A of the torque motor 11.

The invention provides a servo amplifier which will withstand great extremes of shock and vibration. The device consumes little power and dissipates small quantities of heat. It is compact in size, light in weight, and relatively long in life expectancy. The device provides a clean, well-phased signal of proper amplitude and polarity for operation of a torque motor Ill, and also provides a novel method of injecting dither into a discriminating circuit. The circuit is especially useful in missile applications although it can also be used to drive a magnetic amplifier, operate a relay where a reversing direct current is required, or operate a small motor directly.

Although but one embodiment of the invention has been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes can be made in the design and arrangement of the parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.

1. A servo amplifier comprising the combination of a preamplifier receiving an input signal, a discriminator including a pair of transistors connected in push-pull configuration, each of the transistors receiving a signal from the preamplifier, a dither generator applying a signal of small amplitude oscillations to only one discriminator transistor, and means serially coupling each discriminator transistor and a source of electrical energy to a separate winding of a torque motor whereby motor deflection is controlled by the input signal applied to the preamplifier and thereby to said discriminator transistors.

2. A servo amplifier comprising the combination of a discriminator having a pair of transistors connected in push-pull configuration, means applying an input control signal to each of the transistors, a dither generator applying a signal of small amplitude oscillations to only one of the transistors, and means serially coupling each of said transistors and a source of electrical energy to a separate winding of a torque motor, said coupling means applying the output of each of the transistors to the separate winding of the torque motor whereby motor defiection is controlled by the input to the preamplifier.

3. A servo amplifier comprising the combination of a preamplifier receiving an input signal, a discriminator including a pair of transistors, each of the transistors having an emitter and a collector, means applying a signal from the preamplifier to each of the emitters, a dither generator applying a signal of small amplitude oscillations to only one of the emitters, and means including a source of electrical energy for operatively connecting each of the collectors to a separate winding of a torque motor whereby motor rotation is controlled by the input signal to the preamplifier.

4. A servo amplifier comprising the combination of a preamplifier receiving an input signal, a discriminator including a pair of transistors, each of the discriminator transistors having an emitter and a collector and connected in push-pull configuration, means applying a signal from the preamplifier to the emitter of each of the discriminator transistors, an oscillator circuit including a transistor, means connecting the oscillator circuit to the emitter of one of the discriminator transistors, and electrical energy coupling means serially connecting each collector and base of the discriminator transistors to a separate winding of a torque motor in such a manner that energization of each of the separate windings may be effectively controlled by said discriminator transistors, whereby motor deflection is controlled by the input signal to the preamplifier.

5. A servo amplifier comprising the combination of a preamplifier receiving an input signal, a discriminator including a pair of transistors connected in push-pull configuration, each of the discriminator transistors having an emitter and a collector, means coupling the preamplifier to each of the emitters whereby one emitter receives a signal from the preamplifier 180 degrees out of phase with the signal received by the other emitter, a dither generator including a transistor applying a signal of small amplitude oscillations to the emitter of one of the discriminator transistors, and alternating current coupling means serially connecting each collector and base of the disciminator transistors to a separate winding of a torque motor in such a manner that energization of each of the separate windings may be effectively controlled by said discriminator transistors, whereby deflection of the torque motor is controlled by the input signal to the preamplifier.

6. A servo amplifier comprising the combination of a pair of transistors connected in push-pull configuration, a source of alternating current, means for modifying said alternating current and applying an alternating current input control signal to each of the transistors, a dither generator applying a signal of small amplitude oscillations to only one of the transistors, inductive coupling means including an input winding operatively connected to said source of alternating current and an output Winding, and means serially comiected to said output winding for applying the output of each of the transistors together with the alternating current induced in said output winding to a separate winding of a torque motor whereby motor operation is controlled by the input control signal to the preamplifier.

'7. A servo amplifier comprising the combination of a preamplifier receiving an input signal, a discriminator including a pair of transistors, each of the transistors having an emitter and a collector and connected in pushpull configuration, means applying a signal from the preamplifier to the emitters of each of the discriminator transistors, an oscillator circuit including a transistor and .producing small amplitude oscillations, means coupling the small amplitude oscillations to the emitter of one of the discriminator transistors, and means including elec trical energy coupling means serially connecting the collector and base of each of the discriminator transistors to a separate winding of a torque motor in such a manner that energization of said separate windings may be effectively controlled at the collector and base of said discriminator transistors, whereby motor operation is controlled by the input signal to the preamplifier.

8. The servo amplifier of claim 7 wherein the means coupling the small amplitude oscillations to the emitter of one of the discriminator transistors includes adjustable means for controlling the amplitude of oscillations applied to the emitter of the one discriminator transistor.

9. A servo amplifier comprising the combination of a source of alternating current, a preamplifier receiving an input signal derived from said source, a discriminator including a pair of transistors, each of the transistors having an emitter, a collector, and a base, means coupling the preamplifier to each of the emitters whereby one emitter receives an alternating current signal from the preamplifier out of phase with the signal received by the other emitter, a dither generator applying a signal of small amplitude oscillations to only one of the emitters, means connecting each of the collectors to a separate winding of a torque motor, coupling means serially con meeting the bases of said discriminator transistors to each of the separate windings of said torque motor, said serially connecting means coupling said source of alternating current to said separate windings in such a manner that energization of said separate windings may be effectively controlled at the collector and base of said discriminator transistors, whereby motor operation is controlled by the input signal to the preamplifier.

10. A servo amplifier comprising the combination of a preamplifier including a transistor having a base, an emitter and a collector, means coupling an input signal across the emitter and the base of the preamplifier tran sistor, a discriminator including a pair of transistors, each of the discriminator transistors having an emitter, a collector, and a base, means coupling the collector of the preamplifier transistor to each emitter of the discriminator transistors, a dither generator applying a signal of small amplitude oscillations to the emitter of one of the discriminator transistors, means connecting each collector of the discriminator transistors to a separate Winding of a torque motor, and other means for serially coupling a source of electrical energy to the bases of the discriminator transistors and each of the separate windings of the torque motor so that energization of the separate windings by said source may be effectively controlled at the collector and base of the discriminator transistors, whereby motor operation is controlled by the input signal to the preamplifier.

11. A servo amplifier comprising the combination of a preamplifier receiving an input signal, a discriminator including a pair of transistors, each of the transistors having an emitter, a collector, and a base, means coupling the preamplifier to each of the emitters, means coupling each of the collectors to one of a pair of windings of a torque motor for controlling a hydraulic servo valve, a source of alternating current, an induction transformer including a primary winding energized by said source and a secondary winding, means including said secondary winding for connecting each of the bases to the pair of windings, and an oscillator connected to one of the emitters and producing small amplitude oscillations whereby the oscillator signal causes the torque motor to oscillate thereby overcoming st-atic friction in the hydraulic servo valve.

12. A servo amplifier comprising the combination of a source of alternating current, control means for applying from said source an input signal, a preampliler for receiving the input signal, a pair of amplifiers connected in push-pull configuration, means coupling the preamplifier to each of the amplifiers, an oscillator applying a signal of small amplitude oscillations to only one of the pair of amplifiers, and transformer means energized by said source and including output means coupling an output of each of the pair of amplifiers and an output of said transformer means to a separate winding of a torque motor whereby motor operation is controlled by input to the preamplifier and by the signal of small amplitude oscillations applied by the oscillator to only one of said pair of amplifiers.

13. A servo amplifier comprising the combination of a preamplifier for receiving an input signal and arranged to apply an output signal, a pair of amplifiers connected in push-pull configuration, means coupling the output signal from the preamplifier to a first of the pair of amplifiers, an oscillator arranged to apply an oscillating output signal of small amplitude, means coupling the oscillating output signal from the oscillator and an output signal from the preamplifier to a second of the pair of amplifiers, and means serially coupling a source of electrical energy and the output of each of the amplifiers to a separate winding of a torque motor whereby the motor is controlled by the input signal to the preamplifier and by the oscillating output signal of the oscillator.

References Cited by the Examiner UNITED STATES PATENTS 1,999,646 4/1935 Wittkuhus 318-19 2,545,223 3/1951 Briggs 318-29 X 2,688,112 8/1954 Wimberly 318-28 2,786,972 3/1957 Dreier, et al 318 2,995,690 8/ 1961 Lemon 318 MILTON O. HIRSHFIELD, Primary Examiner. 

1. A SERVO AMPLIFIER COMPRISING THE COMBINATION OF A PREAMPLIFIER RECEIVING AN INPUT SIGNAL, A DISCRIMINATOR INCLUDING A PAIR OF TRANSISTORS CONNECTED IN PUSH-PULL CONFIGURATION, EACH OF THE TRANSISTOR RECEIVING A SIGNAL FROM THE PREAMPLIFIER, A DITHER GENERATOR APPLYING A SIGNAL OF SMALL AMPLITUDE OSCILLATIONS TO ONLY ONE DISCRIMINATOR TRANSISTOR, AND MEANS SERIALLY COUPLING EACH DISCRIMINATOR TRANSISTOR AND A SOURCE OF ELECTRICAL ENERGY TO A SEPARATE WINDING OF A TORQUE MOTOR WHEREBY MOTOR DEFLECTION IS CONTROLLED BY THE INPUT SIGNAL APPLIED TO THE PREAMPLIFIER AND THEREBY TO SAID DISCRIMINATOR TRANSISTORS. 